1. Field of the Invention
This invention relates to a new process for the production of polyisocyanates having a biuret structure by reacting aliphatic or cycloaliphatic diisocyanates with sub-stoichiometric quantities of certain aldoximes.
2. Description of the Prior Art
Polyisocyanates having a biuret structure are known. They are obtained, for example, by reacting aromatic or aliphatic polyisocyanates with water, inorganic salts containing water of crystallization or organic compounds containing water of hydration as described in U.S. Pat. No. 3,124,605 or with hydrogen sulphide as described in German Pat. No. 1,165,580, via the intermediate stage of a urea diisocyanate. These known processes have the disadvantage that undesirable, insoluble polyisocyanates containing urea groups are always formed, precipitating as solids and thus blocking, for example, the apparatus used for producing the polyisocyanates containing biuret groups.
It is also known that polyisocyanates having a biuret structure can be obtained by reacting aromatic or aliphatic diisocyanates with monohydric tertiary alcohols as described in U.S. Pat. No. 3,358,010. The disadvantage of this known process lies both in the high reaction temperature required, which is in the range from about 180.degree. to 200.degree. C., and, above all, in the fact that readily inflammable isobutylene gas is formed during the reaction and can only be removed at considerable expense.
New biuretizing agents have now suprisingly been found which makes it possible for high-grade polyisocyanates having a biuret structure to be produced without any of the disadvantages of the known processes. In the context of the invention, the term "biuretizing agent" is used for compounds which react at elevated temperature with organic isocyanates to form compounds containing biuret groups. Thus, water, hydrogen sulphide or tet.-butanol for example are the "biuretizing agents" used in the known processes mentioned above. The new biuretizing agents according to the invention are organic compounds containing aldoxime groups. Where they are used for the production of biuret-group-containing polyisocyanates based on aliphatic or cycloaliphatic diisocyanates, insoluble urea diisocyanates are surprisingly not formed at any stage of the reaction, although the reaction according to the invention is carried out at comparatively low temperatures. In addition, only the nitriles corresponding to the biuretizing agents i.e. substances which are easy to handle and distill, are formed as secondary products apart from readily absorbable carbon dioxide. These nitriles correspond to the residue of the biuretizing agents used, which is inert with respect to the biuretizing reaction.